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Why Test your Solder

Why Test your Solder?

 

During soldering the liquid solder in the solder bath will become contaminated by impurity elements from the melting pot and the parts to be soldered. These contaminants will impair the solder quality of your processes. Over the period of some use, a contaminant load (equilibrium) will form in the alloy make up of your solder. This is a variable state that depends on: 

  • Solder use and consumption type (Wave, Dip or Selective). 
  • Frequency of solder replenishment. 
  • Components Finishes and exposure time.  

In case of a change of the materials to be soldered (e.g. board metallization changed from Cu to NiAu) the equilibrium will readjust itself and a different contaminant load at new concentrations will be present in new concentrations. Regular monitoring of the composition and impurities of the solder is a basic prerequisite to achieve a high degree of process stability, since deviations can be detected solder bath alloy metals can be added and soldering defects prevented. This is more important in case of lead-free soldering than it is with conventional SnPb solders since copper is dissolved at a much faster rate by lead-free solders. Furthermore the level of lead contamination must be controlled under 0.1% in case of ROHS-compliant production. 

Below you will find the most common defects associated with Solder Contaminant Load:

 

    Typical Sn63Pb37 Solder Pot 

Contaminant 

 Maximum Allowed
(J-STD 001E)

Source of Contamination 

Defect that can be Created 

Copper  (Cu)

 0.3

Exposed copper component leads or un-plated copper pads

Can cause excessive dross build up and cause bridging and poor wetting 

Gold (Au)

 0.2

Gold plated pads or component leads

Solder joint strength may become compromised 

 Cadmium (Cd)

 0.005

Cadmium coated fixtures or shields

ROHS Directive Maximum allowed

 Zinc  (Zn)

 0.005

Heat sinks or Zinc Plated Bolts

Can cause bridging, icicling and surface oxidation

Aluminum (Al) 

 0.006

Heat Sinks or Fixtures

Can cause bridging, icicling and surface oxidation

 Antimony (Sb)

0.5

Found in some solder additives

Can cause thermal fatigue and produce brittle joints

 Arsenic (As)

 0.03

Can be found in recycled solder

Solder joint de-wetting 

 Bismuth (Bi)

 0.25

Some Lead Free solders contain Bismuth

Poor wetting

 Silver (Ag)

 0.1

Lead free solder was added to the pot

Poor solder performance

 Nickel (Ni)

0.01

Stainless steel can elevate Nickel percentages

Excessive solder dross and poor solder activity